Reduced Symmetry Enhances Nanoparticles

Plasmon resonance, a governing factor in optical properties of metallic nanoparticles, relies heavily on particle geometry. Researchers at Rice University in Houston altered the spherically symmetric geometry of a nanoshell to explore the effect of reduced symmetry on plasmon modes.

The reduced-symmetry nanoparticles, dubbed nanoeggs, were fabricated by an anisotropic electroless plating technique. Silica-gold nanoshells were immobilized on a glass slide and immersed in a solution that initiated the plating of gold onto the exposed surfaces of the particles, effectively offsetting the core from the center of the shell.

A nanoshell consists of a dielectric core surrounded by a concentric metal shell. Plasmons on the inner and outer surfaces interact to form two hybridized plasmons; however, this occurs only between two plasmons occupying the same angular momentum state.

As the researchers reduced the symmetry, plasmons of all angular momenta interacted and contributed to the creation of the hybridized plasmon modes. Consequently, all plasmon modes could be optically excited, resulting in a broadened optical spectrum and an enhanced electromagnetic field on the surface of the nanoegg where the shell is thinnest (see figure).

Near-field plots of silica-gold nanoshells with increasing core offsets exhibit electromagnetic field enhancements on the thinnest surface of the shell. The core is offset (from left to right) 0, 4.5 and 7.5 nm, with corresponding field enhancements of 13.2, 25.2 and 60.3. Courtesy of PNAS.The observed increase in spectral complexity and evolution of optical features agreed well with the researchers’ predictions. They believe that the approach will prove useful for analyzing the near- and far-field optical responses of reduced-symmetry nanoparticles that are even more complex than nanoeggs. Ultimately, they hope to design nanoparticle geometries with specific near-field optical properties that may be used in a variety of applications, including sensing and biomedicine.